Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as In-Band On-Channel (IBOC) digital audio broadcasting (DAB), uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
Both AM and FM In-Band On-Channel (IBOC) hybrid broadcasting systems utilize a composite signal including an analog modulated carrier and a plurality of digitally modulated subcarriers. Program content (e.g., audio) can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers. The analog audio is delayed at the transmitter by a diversity delay. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
The digital signal is delayed in the receiver with respect to its analog counterpart such that time diversity can be used to mitigate the effects of short signal outages and provide an instant analog audio signal for fast tuning. Hybrid-compatible digital radios incorporate a feature called “blend” which attempts to smoothly transition between outputting analog audio and digital audio after initial tuning, or whenever the digital audio quality crosses appropriate thresholds.
In the absence of the digital audio signal (for example, when the channel is initially tuned) the analog AM or FM backup audio signal is fed to the audio output. When the digital audio signal becomes available, the blend function smoothly attenuates and eventually replaces the analog backup signal with the digital audio signal while blending in the digital audio signal such that the transition preserves some continuity of the audio program. Similar blending occurs during channel outages which corrupt the digital signal. In this case the analog signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog when the digital corruption appears at the audio output.
Blending will typically occur at the edge of digital coverage and at other locations within the coverage contour where the digital waveform has been corrupted. When a short outage does occur, as when traveling under a bridge in marginal signal conditions, the digital audio is replaced by an analog signal.
When blending occurs, it is important that the content on the analog audio and digital audio channels is time-aligned to ensure that the transition is barely noticed by the listener. The listener should detect little other than possible inherent quality differences in analog and digital audio at these blend points. If the broadcast station does not have the analog and digital audio signals aligned, then the result could be a harsh-sounding transition between digital and analog audio. This misalignment or “offset” may occur because of audio processing differences between the analog audio and digital audio paths at the broadcast facility.
The analog and digital signals are typically generated with two separate signal-generation paths before combining for output. The use of different audio-processing techniques and different signal-generation methods makes the alignment of these two signals nontrivial. The blending should be smooth and continuous, which can happen only if the analog and digital audio are properly aligned.
The effectiveness of any digital/analog audio alignment technique can be quantified using two key performance metrics: measurement time and offset measurement error. Although measurement of the time required to estimate a valid offset can be straightforward, the actual misalignment between analog and digital audio sources is often neither known nor fixed. This is because audio processing typically causes different group delays within the constituent frequency bands of the source material. This group delay can change with time, as audio content variation accentuates one band over another. When the audio processing applied at the transmitter to the analog and digital sources is not the same—as is often the case at actual radio stations—audio segments in corresponding frequency bands have different group delays. As audio content changes over time, misalignment becomes dynamic. This makes it difficult to ascertain whether a particular time-alignment algorithm provides an accurate result.
Existing time alignment algorithms rely on locating a normalized cross-correlation peak generated from the analog and digital audio sample vectors. When the analog and digital audio processing is the same, a clearly visible correlation peak usually results.
However, techniques that rely solely on normalized cross-correlation of digital and analog audio vectors often produce erroneous results due to the group-delay difference described above. When the analog and digital audio processing is different, the normalized cross correlation is often relatively low and lacks a definitive peak.
Although multiple measurements averaged over time can reduce the dynamic offset measurement error, this leads to excessive measurement times and potential residual offset error due to persistent group-delay differences. Since an HD Radio receiver may use this measurement to improve real-time hybrid audio blending, excessive measurement time and offset error make this a less attractive solution. Therefore, improved techniques for measuring time offsets are desired.